1. Field of Invention
The invention relates to agricultural tractors and particularly to multi ratio power take-off (PTO) drive systems fitted thereto.
2. Description of Related Art
Agricultural tractors have for many decades included PTO systems which comprise a splined PTO stub to which an implement drive shaft is attached and from which the implement can derive torque. Example implements which typically require a PTO drive include balers, power harrows and sprayers. A direct mechanical connection normally exists between the tractor engine and the PTO stub with a clutch typically provided to selectively engage the PTO output.
Today, the majority of tractor mounted implements operate at the original ISO 500 industry standard PTO speed of 540 rpm. A PTO stub meeting this standard has 6 splines. Some implements, however, are more power hungry than others and more recently have been designed to operate at a faster industry standard of 1000 rpm thus providing greater efficiency. A PTO stub meeting the standard for this faster speed has 20 splines.
To cater for different implements having different operating requirements, modern tractors are fitted with multi-ratio PTO drive systems which offer PTO output speeds of 540 rpm and 1000 rpm at the rated engine speed of the tractor. The different design of PTO stubs for the different operating speeds is intended to safeguard against inadvertent attachment of an implement designed to operate at 540 rpm to a tractor set up to deliver 1000 rpm.
Further to providing 540 rpm and 1000 rpm at the rated engine speed, further ratios are provided in some cases to give 540 rpm and/or 1000 rpm at a lower engine speed, commonly referred to a 540E mode and a 1000E mode respectively, ‘E’ representing economy.
Despite this difference in PTO stub design, there is still a reliance upon the operator to activate the correct PTO output speed for the implement attached thus leaving open the risk of incorrect speed ratio selection and resulting damage to the PTO drive system and/or implement.
Known multi-ratio PTO drive systems include a mechanical selector mechanism connected to a lever in the driver's cab from where the operator can select the required drive ratio. Such a mechanism is deemed cumbersome and requires space in the cab to accommodate the lever. Furthermore, the linkages involved are susceptible to wear and failure.